SSM aluminum alloy castings outperform, in both cost and performance, other casting techniques, such as conventional die casting, which is performed under high pressure, gravity permanent mold casting and squeeze casting. SSM casting methods, when utilized for the manufacturing of aluminum alloy products/castings, have proven advantageous over other casting techniques because SSM castings tend to exhibit higher mechanical properties in the areas of strength and ductility and reduced porosity than castings produced by the above-listed other methods.
Microstructures of SSM aluminum alloy castings reveal the primary phase particles as round crystals that are often referred to as rosettes or globules. The primary phase particles in SSM aluminum alloy castings that contain less than twelve percent silicon are comprised of essentially aluminum. Because solid primary phase particles are part of the semi-solid metal being injected into a mold/die cavity, the microstructure of the primary phase of an aluminum alloy prior to injection into a mold/die is indicative of the microstructure of the primary phase of the resulting aluminum alloy casting. Thus, when SSM methods of casting are utilized, the mechanical properties of a casting can be predicted before a casting is even produced. Accordingly, the production of castings with defects can be avoided.
Unlike SSM methods of casting, non-SSM casting processes involve injection/pouring of a molten metal directly into the die. Thus, because the metal is molten, with no parts of the metal being solid, the microstructure of the resulting casting cannot be ascertained until after the molten metal has solidified in the mold/die cavity. Thus, with non-SSM casting processes the microstructure of the primary phase of a casting cannot be predicted before the casting is formed.
Typically, the microstructures of castings prepared by non-SSM casting processes have dendrites. Dendrites are “tree-like ” structures, and castings with dendrites are prone to microporosity and have inferior mechanical properties than those that exhibit round crystals.
Thixocasting and Rheocasting are SSM methods of casting. Thixocasting involves the electromagnetic stirring of metal during solidification/freezing to provide aluminum SSM feedstock billets up to approximately 4″ in diameter. The stirring action due to the movement of liquid fragments the aluminum dendrites as they form during solidification and results in the formation of small equiaxed grains in the billets. The billets are subsequently cut into slugs, and re-heated to a semi-solid state before being injected into the cavity. It is during the billet re-heating stage that the equiaxed aluminum grains undergo globularization. Chemically grain-refined billets are often used in lieu of electromagnetically stirred billets. Heating of grain-refined billets in the semi-solid temperature regime also helps yield globular primary phase particles prior to injection into die cavity.
Rheocasting, which is also known as “slurry ” or “slurry-on-demand ” casting, involves heating a metal to a liquid state, cooling the molten metal to a semi-solid state, and then injecting the semi-solid metal into the die cavity. Rheocasting is more efficient than Thixocasting because Rheocasting involves fewer steps than Thixocasting.
Rheocasting has also proven to be more economically feasible than Thixocasting because any unused scrap metal can be easily re-melted and reprocessed by an SSM component manufacturer. With Thixocasting, the scrap metal has to be reformed into billets by the billet manufacturer via the use of electromagnetic stirring or chemical grain refining before being used again. However, unlike Thixocasting, Rheocasting requires only that the scrap metal is re-melted and cooled to a semi-solid state before it is injected into a die cavity. As a result, scrap metal can easily be reused with the Rheocasting method of SSM casting and the expense associated with recycling scrap metal is less.
In recent years, SSM casting methods utilizing aluminum alloys have been used for manufacturing brake cylinders, fuel rails, engine brackets steering knuckles, suspension links and auto seat backs because, in addition to the above-discussed advantages over non-SSM casting techniques, SSM casting methods offer non-turbulent filling (i.e., less air entrapment), require lower die temperatures, reduce cycle time, reduce shrinkage, and provide the option of heat treatment (i.e., solution treatment).
Among the various casting alloys in use, A356.2 and 357 are the primary aluminum alloys used for SSM castings, including castings of automotive components. The chemistries of A356.2 and 357 are as follows:
A356.2               357Percent ofPercent ofElementWeightElementWeightSilicon6.5-7.5Silicon6.5-7.5Iron0.12 maxIron0.12 maxManganese0.05 maxManganese0.03 maxMagnesium0.30-0.45Magnesium0.45-0.6Zinc0.50 maxZinc0.05 maxTitanium0.20 maxTitanium0.20 maxCopper0.10 maxCopper0.03 maxOthers0.15 totalOthers0.15 totalAluminumBalanceAluminumBalance
A356.2 and 357, when used with SSM casting methods, generate castings of essentially high toughness, i.e., ability to absorb energy before failure, and thus, have been found suitable for automotive components, such as steering knuckles and suspension links.
However, the A356.2 and 357 alloys, when used with SSM casting methods, have not been found suitable for automotive components that require essentially high strength, i.e., high load bearing ability, such as axle carriers, rack and pinion housings, and steering column housings.
There are, however, problems associated with the use of A356.2 and 357 aluminum alloys. Maintaining low percentages of iron, copper and zinc increases the cost of the A356.2 and 357 aluminum alloys. By keeping the iron content low, there is also the potential that soldering will occur during the casting process. Soldering refers to the phenomenon that takes place when aluminum adheres to the die cavity during the die casting process. Soldering occurrences often lead to defective castings.
Accordingly, it is desirable to provide an aluminum alloy that can be utilized with SSM methods of castings, especially with the increasingly popular Rheocasting method of SSM casting, that can produce high integrity, high-strength automotive components, such as axle carriers, rack and pinion housings, and steering column housings.
Further, it is desirable to provide an aluminum alloy that is less prone to the soldering phenomenon.
It is also desirable to provide an alloy that is less expensive to produce than other alloys such as A 356.2 and 357.